Semiconductor device and manufacturing method thereof, delamination method, and transferring method

ABSTRACT

A substrate and a delamination film are separated by a physical means, or a mechanical means in a state where a metal film formed over a substrate, and a delamination layer comprising an oxide film including the metal and a film comprising silicon, which is formed over the metal film, are provided. Specifically, a TFT obtained by forming an oxide layer including the metal over a metal film; crystallizing the oxide layer by heat treatment; and performing delamination in a layer of the oxide layer or at both of the interface of the oxide layer is formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a delamination method of a functionalthin film, particularly to a delamination method of a film or a layereach of which is provided with various elements. In addition, thepresent invention relates to a transferring method for pasting aseparated film to a film substrate, and further relates to asemiconductor device comprising a thin film transistor (hereinafterreferred to as a TFT), which is formed in accordance with thetransferring method, and to a manufacturing method thereof.

2. Description of the Related Art

Recently, a technique for forming a TFT by using a semiconductor thinfilm (with a thickness of about several nanometers to several hundrednanometers) formed over a substrate provided with an insulating surfaceis attracting attention. A TFT is widely applied to an electronic devicesuch as an IC or an electro-optic device, and is developed especially asa switching element or a driver circuit of a display device.

Such display devices can be mass-produced by performing dicing forobtaining multiple panels. Glass substrates and quartz substrates aremostly used; however, they have disadvantages of fragility and heavinessto be enlarged. Therefore, forming a TFT element on a flexible substratetypified by a flexible plastic film is being tested.

However, when a sophisticated polysilicon film is used for an activelayer of a TFT; a process at a high temperature at several hundreddegrees centigrade is necessary in a manufacturing process, so that thepolysilicon film can not be formed directly on a plastic film.

Therefore, a method of separating a delamination layer from thesubstrate by using a separation layer in between is proposed. Forexample, a separation layer comprising such as amorphous silicon, asemiconductor, nitride ceramics, or an organic polymer is provided andexposed to a laser beam through the substrate; the substrate isseparated by a delamination or the like in the separation layer(Reference 1: Japanese Patent Laid-open Publication No. 10-125929). Inaddition, a reference describes an example of completing a liquidcrystal display device by pasting a delamination layer (referred to as alayer to be transferred) to a plastic film (Reference 2: Japanese PatentLaid-open Publication No. 10-125930). Techniques of respective companiesare introduced in the articles on flexible displays (Reference 3: NikkeiMicrodevices, Nikkei Business Publications, pp. 71-72, Jul. 1, 2002).

However, in the method described in the above publications, it isrequired to use a substrate which is highly transparent to light.Further, a rather high-energy laser beam is necessary for impartingsufficient energy to release hydrogen included in amorphous siliconthrough a substrate. That causes a problem of damage to a delaminationlayer. Further, the above publication describes a structure in which alight-resistant layer or a reflective layer is provided to prevent thedamage to the delamination layer; however, in that case, it is difficultto fabricate a transmissive liquid crystal display device or a lightemitting device which emits light downward. Still further, with theabove method, it is difficult to separate a delamination layer having alarge area.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems and itis an object of the present invention to provide a technique forperforming separation between a substrate and a delamination layer by aphysical means or a mechanical means in a state where a metal filmformed over a substrate, and a delamination layer comprising an oxidefilm including the aforementioned metal and a film comprising silicon,which is formed over the metal film, are provided. Specifically, a TFTobtained by forming an oxide layer including the aforementioned metalover a metal film; crystallizing the aforementioned oxide layer by heattreatment; and performing delamination in a layer of the oxide layer orat the interfaces of both surfaces of the aforementioned oxide layer isformed.

A TFT formed according to the present invention can be applied to anylight emitting device of top emission type or bottom emission type; orto any liquid crystal display device of transmissive type, reflectivetype, or semi-transmissive type; or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E show a delamination process according to the presentinvention.

FIG. 2 shows an experimental sample in the present invention.

FIGS. 3A and 3B show a TEM picture and the frame format of experimentalsample A in the present invention.

FIGS. 4A and 4B show a TEM picture and the frame format of experimentalsample B in the present invention.

FIGS. 5A and 5B show a TEM picture and the frame format of experimentalsample C in the present invention.

FIGS. 6A and 6B show a TEM picture and the frame format of experimentalsample D in the present invention.

FIGS. 7A and 7B show a TEM picture and the frame format of experimentalsample E in the present invention.

FIGS. 8A and 8B are a figure showing EDX spectrum and a quantitativeresult of experimental sample A in the present invention.

FIGS. 9A and 9B are a figure showing EDX spectrum and a quantitativeresult of experimental sample B in the present invention.

FIGS. 10A and 10B are a figure showing EDX spectrum and a quantitativeresult of experimental sample C in the present invention.

FIGS. 11A to 11D show experimental samples in the present invention.

FIGS. 12A and 12B show a TEM picture and the frame format ofexperimental sample 1 in the present invention.

FIGS. 13A and 13B show a TEM picture and the frame format ofexperimental sample 2 in the present invention.

FIGS. 14A and 14B show a TEM picture and the frame format ofexperimental sample 3 in the present invention.

FIGS. 15A and 15B show a TEM picture and the frame format ofexperimental sample 4 in the present invention.

FIGS. 16A to 16C show XPS measurements of experimental samples A to C inthe present invention.

FIGS. 17A to 17F are figures in which XPS measurements shown in FIGS.16A to 16C are standardized.

FIGS. 18A to 18C show XPS measurements of experimental samples A to C inthe present invention.

FIGS. 19A and 19B show a TEM picture and the frame format of thesubstrate side after the separation according to the present invention.

FIGS. 20A and 20B show a TEM picture and the frame format of thesemiconductor film side after separation according to the presentinvention.

FIG. 21 shows SIMS of sample A in the present invention.

FIG. 22 shows SIMS of sample B in the present invention.

FIG. 23 shows SIMS of sample C in the present invention.

FIGS. 24A and 24B show XPS measurements after the separation accordingto the present invention.

FIGS. 25A and 25B show waveform analysis of XPS measurements shown inFIGS. 24A and 24B.

FIGS. 26A and 26B show a light emitting device formed according to thepresent invention.

FIGS. 27A and 27B show a liquid crystal display device formed accordingto the present invention.

FIG. 28 shows a CPU formed according to the present invention.

FIGS. 29A to 29E show electronic devices formed according to the presentinvention.

FIGS. 30A and 30B show experimental results of the present invention.

FIG. 31 shows an experimental result of the present invention.

FIG. 32 shows an experimental result of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, a description is made to explainembodiment modes according to the present invention.

Embodiment Mode 1

First, a metal film 11 is formed on a first substrate 10 as shown inFIG. 1A. Note that, any substrate that has rigidity for withstanding thelamination process thereafter, for example, a glass substrate, a quartzsubstrate, a ceramic substrate, a Si substrate, a metal substrate, or astainless substrate, can be used for the first substrate. An elementselected from the group consisting of W, Ti, Ta, Mo, Nd, Ni, Co, Zr, Zn,Ru, Rh, Pd, Os, and Ir; a single layer mainly comprising an alloymaterial or a compound material thereof; or a lamination thereof, can beused for the metal film. The metal film may be formed over the firstsubstrate by sputtering using a metal as a target. Note that, the filmthickness of the metal film shall be 10 nm to 200 nm, preferably be 50nm to 75 nm.

Instead of a metal film, a metal film which is nitrided (metal nitridefilm) may be used. Nitrogen and oxygen may be added further to the metalfilm. For example, ion implantation of nitrogen or oxygen to metal filmmay be performed, or the metal film may be formed by sputtering in afilm formation chamber which shall be a nitrogen or oxygen atmosphere.Furthermore, nitride metal may be used as a target.

Hereupon, when a metal alloy of the aforementioned metal (for example,an alloy of W and Mo: W_(x)Mo_(1-x)) is used for the metal film, pluraltargets such as a first metal (W) and a second metal (Mo), or a targetof an alloy of the first metal (W) and the second metal (Mo) is disposedin a film formation chamber, thereby forming the metal film bysputtering.

When the metal film is formed by sputtering, the film thickness of theperiphery of the substrate occasionally becomes uneven. Therefore, it ispreferable to remove a film of the periphery portion by dry etching; onthat occasion, an insulating film such as a SiON film or a SiNO film maybe formed into approximately 100 nm between the first substrate 10 andthe metal film 11 so that the first substrate is not etched.

By setting the formation of metal film appropriately as above, thedelamination process can be controlled, and a range of process isbroadened. For example, when a metal alloy is used, use or diuse of heattreatment as well as the temperature of heat treatment can be controlledby controlling composition ratio in each metal of the alloy.

A delamination layer 12 is formed over the metal film 11. Thedelamination layer has an oxide film for forming an oxide layerincluding the aforementioned metal over the metal film 11, and asemiconductor film. The semiconductor film of the delamination layer maybe in a state where a TFT, an organic TFT, a thin film diode, aphotoelectric transducer comprising PIN junctions of Si, a siliconresistor, a sensor element (typically, a pressure-sensitive fingerprintscanner using polysilicon) or the like is formed in a desiredmanufacturing process.

Silicon oxide, silicon oxynitride or the like can be formed by asputtering method or a CVD method as the oxide film. Incidentally, athickness of the oxide film is preferably two times larger than that ofthe metal film 11. Here, by sputtering using a Si target, a siliconoxide film is formed with a film thickness of 150 nm to 200 nm.

In the present invention, when an oxide film is formed, the oxide layerincluding the aforementioned metal is formed over the metal film (notillustrated). The oxide layer may be formed into a film thickness of 0.1nm to 1 μm, preferably 0.1 nm to 100 nm, further preferably 0.1 nm to 5nm.

Another method for forming the oxide layer aside from the above can usea thin oxide film formed by processing an aqueous solution havingsulfuric acid, hydrochloric acid or nitric acid; an aqueous solution inwhich sulfuric acid, hydrochloric acid or nitric acid and hydrogenperoxide water are mixed; or ozone aqua. As yet another alternative,oxidation may be performed by plasma treatment in an oxygen atmosphereor by generating ozone with ultraviolet irradiation in an oxygencontaining atmosphere, or, a thin oxide film may be formed by heatingapproximately at 200° C. to 350° C. with a clean oven.

In the delamination layer 12, it is preferable that an insulating filmcomprising a nitride such as SiN, or SiON be provided as a base filmparticularly under a semiconductor film to prevent immersion ofimpurities or garbage penetrating the exterior of a metal film or asubstrate.

Thereafter, heat treatment is performed at 380° C. to 410° C., 400° C.,for example. By the heat treatment, the oxide layer is crystallized, andthe hydrogen contained in the delamination layer 12, particularly,hydrogen of a semiconductor film is diffused. Heat treatment in aprocess for manufacturing a semiconductor device may be performedconcurrently with heat treatment for the above step thereby reducing thenumber of the processes. When an amorphous semiconductor film is formedand a crystalline semiconductor film is formed by using a heatingfurnace or by laser irradiation, heat treatment at least at 500° C. maybe performed for the crystallization, thus diffusing hydrogen as well asforming a crystalline semiconductor film.

A second substrate 13 which fixes the delamination layer 12 is pasted tothe delamination layer 12 with a first adhesive material (bindingmaterial) 14 as shown in FIG. 1B. Note that, it is preferable that asubstrate with rigidity which is higher than that of the first substrate10 be used for the second substrate 13. For example, a peelable adhesivesuch as ultra violet peelable adhesive, which is removed with ultraviolet rays, or a heat peelable adhesive, which is removed with heat; awater-soluble adhesive; or a two-sided tape may preferably be employedfor the first binding material 14.

Next, the first substrate 10 provided with the metal film 11 isseparated by using physical means (FIG. 1C). The separation occurs in alayer of the crystallized oxide layer or at the interfaces of bothsurfaces of the oxide layer, that is, at the interface between the oxidelayer and the metal film or at the interface between the oxide layer andthe delamination layer; although it is not illustrated since a figureshows a frame format. Thus, the delamination layer 12 can be separatedfrom the first substrate 10.

As shown in FIG. 1D, the separated delamination layer 12 is pasted tothe third substrate 16 which is to be a transfer body, by the secondbinding material 15. An ultraviolet curable resin such as an epoxy resinadhesive, a resin additive, a two-sided tape, or the like may be usedfor the second binding material 15. Note that, when the surface of thethird substrate is adhesive, the second binding material may notnecessarily be used. Further, the side surfaces of the delaminationlayer 12 may also be covered with the third substrate. A substrate withflexibility and thin film thickness (such a substrate is hereinafterreferred to as a film substrate), for example, a plastic substrate suchas a substrate of polycarbonate, polyarylate, polyethersulfone;polytetrafluoro-ethylene substrate; or ceramic substrate may be used forthe third substrate 16.

Subsequently, the first binding material 14 is removed and the secondsubstrate 13 is delaminated (FIG. 1E). Specifically, ultravioletirradiation, heat treatment, or water washing may be performed to peelthe first binding material. Further, it is preferable to perform plasmacleaning using argon gas and oxygen gas, or bellclean cleaning.

Plural delamination layers provided with TFTs that suit each usage maybe transferred to the third substrate which is to be a transfer body.For example, a delamination layer of a TFT for a pixel area and a TFTfor a driver circuit may be formed, and transferred to a predeterminedarea of the third substrate thereafter.

A TFT and the like which are formed on the film substrate obtained asabove can be employed as a semiconductor element of a light emittingdevice or of a liquid crystal display device.

A light emitting device is formed by forming a light emitting element onthe delamination layer 12 and forming a protective film which is to bean encapsulant thereafter. When a light emitting element is formed onthe delamination layer 12, since the film substrate provided with a TFTis flexible, the delamination layer may be fixed to another glasssubstrate using a binding material such as a tape thereby forming eachlight emitting layer by vacuum deposition. Note that, it is preferablethat a light emitting layer, an electrode and a protective film aresequentially formed without being exposed to the atmosphere.

The order for making a light emitting device is not limitedparticularly, and the following order may be adopted: a light emittingelement is formed over a delamination layer; the second substrate isadhered; the delamination layer having light emitting element isseparated, and it is pasted to the film substrate which serves as thethird substrate. Further, after the formation of the light emittingelement, the whole device may be wrapped in a film substrate designedlarger, which serves as the third substrate.

When a liquid crystal display device is manufactured, a countersubstrate is adhered with a sealing material after the separation of thesecond substrate, and a liquid crystal material may be injected inbetween. The order for making a liquid crystal display device is notlimited particularly, and the following order may also be employed: thesecond substrate is adhered as a counter substrate; the third substrateis adhered; and a liquid crystal is injected in between, may beemployed.

When a liquid crystal display device is manufactured, generally, spacersare formed or sprinkled to maintain a substrate gap; however, spacerswith around 3 times the amount may be formed or sprinkled to maintain agap between a flexible substrate and the counter substrate. Further, thespacers are preferably formed more softly than that are applied to ageneral glass substrate. Still further, it is necessary to fix thepacers so as not to move since a film substrate is flexible.

By applying such a delamination method, a TFT and the like can be formedon a flexible film substrate achieving delamination in the whole surfaceand high yield. In addition, a burden caused by a laser or the like arenot placed on a TFT in the present invention. Thus, a light emittingdevice, a liquid crystal display device, or other display devices, whichhave the TFT and the like becomes thin, hard to be broken even if itdrops, and lightweight. Further, display on a curved surface or inodd-shape becomes possible. A TFT provided on a film substrate, which isformed according the present invention can achieve the enlargement ofdisplay units as well as mass production. The present invention enablesthe recycling of the first substrate and achieves reducing costs of adisplay unit by employing a low-cost film substrate.

Embodiments

A experimental result of the present invention, a light emitting devicemanufactured according to the present invention, a liquid crystaldisplay device, and other electronic devices will be described below.

[Embodiment 1]

In this embodiment, a result of a delamination experiment and an auditobservation of a transmission electron microscope (TEM) will bedescribed.

First, as to a sample shown in FIG. 2, an AN 100 glass substrate(126×126 mm²) as a substrate and a film mainly consisting of tungsten(hereinafter referred to as a W film) deposited by sputtering as a metalfilm are laminated. Thereafter, a SiO₂ film deposited by sputtering as aprotective film forming a delamination layer, a SiON film deposited byCVD as a base film, and an amorphous silicon film deposited by CVD as asemiconductor film are laminated thereover.

Among the above samples, one to which heat treatment is not performedshall be A, another to which heat treatment at 220° C. for one hour isperformed shall be B, and the other to which heat treatment at 500° C.for one hour and heat treatment at 550° C. for four hours thereafter areperformed shall be C. Each of the samples is observed with a TEM. Theresults are shown in FIGS. 3A to 5A. The frame formats corresponding tothe respective TEM pictures (TEM images) are shown in FIGS. 3B to 5B.

It is found that a layer is formed at an interface between W filmserving as a metal film 202 and a protective film 203. Note that, thelayer is not always a complete layer, and is scattered in some cases.

An EDX measurement is performed to specify the composition of the layer.The spectrum and the quantitative result of the EDX measurement onsamples A to C are shown in FIGS. 8A to 10B. Note that, the peaks of Aland Mo are due to the sample fixing holder during the measurement. Theresults in FIGS. 8A to 10B show the existence of tungsten and oxygen inthe layer (hereinafter referred to as a oxide layer).

In comparing TEM pictures in FIGS. 3A to 5A, the oxide layer of sample Cis found to have a crystalline lattice arranged in a specific direction.It is also found that the oxide layers of samples A and B have filmthicknesses of approximately 3 nm; meanwhile, the oxide layer of sampleC is formed to have a rather thinner thickness (3 nm at most).

Such results of the delamination experiment on samples A to C revealthat only sample C in which the oxide layer has a crystalline latticecan be separated.

FIGS. 6A and 7A show TEM pictures of a sample shown in FIG. 2 after heattreatment at 400° C. for one hour, which is to be sample D and a sampleshown in FIG. 2 after heat treatment at 430° C. for one hour, which isto be sample E. FIGS. 6B and 7B show frame formats corresponding to therespective TEM pictures. Note that, the temperature 400° C. which isapplied to sample D is expected to be a boundary temperature ofcrystallization; that is a boundary temperature that can causeseparation.

FIGS. 6A and 6B show that a crystalline lattice is formed on a part ofthe oxide layer in sample D and a crystalline lattice is formed whollyover the oxide layer in sample E.

As a result of the lamination experiments of the above samples D and E,only sample E is found to be separated.

The results of the above delamination experiment and TEM pictures revealthat an oxide layer is formed at the interface between a metal film anda protective film, and that the crystallization of the oxide film beginsto occur approximately at 400° C. When the oxide layer hascrystallinity, it is considered to be a state where separation mayoccur. Namely, it is found that an oxide film over a metal film,specifically, an oxide layer comprising W provided over a W film, needsto be formed.

Accordingly, since separation is possible in a sample wherein an oxidelayer is crystallized, when the oxide film is crystallized by heattreatment, a crystal distortion, a lattice defect (point defect, linedefect, plane defect (for example, plane defect due to crystallographicshear plane which is formed with congeries of oxygen vacancy), anexpansion defect) generate, and separation is considered to occur formthe interfaces thereof.

[Embodiment 2]

Next, a delamination experiment is carried out under a differentcondition for manufacturing the protective film, or as the absence orpresence of the protective film on a W film is varied.

As shown in FIGS. 11A to 11D, the followings are prepared: sample 1formed by sequentially laminating a SiON film 301 formed on a substrate300 by CVD and a W film 302 formed by sputtering (FIG. 11A); sample 2comprising a Si film 303 over a W film, formed by sputtering using argongas, as a protective film (FIG. 11B); sample 3 comprising a SiO₂ film304 formed by sputtering using argon gas and oxygen gas instead of Sifilm (FIG. 11C); and sample 4 comprising a SiO₂ film 305 formed by CVDusing silane gas and nitrogen gas (FIG. 11D).

FIGS. 12A to 15A show TEM pictures of cross sections of respectivesamples 1 to 4. The frame formats corresponding to the respective TEMpictures are shown in FIGS. 12B to 15B.

As illustrated in FIGS. 12A to 15A, an oxide layer is formed over a Wfilm in sample 3; however, an oxide layer is not formed in the othersamples. Note that, a natural oxide film is formed in sample 1; however,the film thickness is so thin that the film is not clearly shown in theTEM picture.

The oxide layer is considered to be formed over the W film due to theoxygen gas employed when the sample 3 is formed. On the other hand, whenthe protective film is formed in sample 2, it is considered that onlyargon gas is used, so that an oxide layer is not formed on the W film.When the film thickness is considered, the oxide layer formed in sample3 is thought to be different from the natural oxide film formed insample 1. It is conceivable that the oxide layer is formed when theprotective film begins to be formed.

Further as to sample 4, the SiO₂ film is formed on a W film by CVD, bywhich an oxide layer may be formed; however, an oxide layer is notobserved as shown in FIG. 15A.

Sample 3 and sample 4 in which oxide layers are formed are consideredhere. The silane gas employed by CVD, by which the SiO₂ film of sample 4is formed contains hydrogen compared with the source gas used in amanufacturing process of the SiO₂ film in sample 3. Namely, an oxidelayer is projected not to be formed in sample 4 due to the presence ofhydrogen. Accordingly, a state in sample 4 can be thought to be varieddue to the hydrogen although an oxide layer is formed on the W film.

As a result of the above, it is conceivable that an oxide layer which isdifferent from a natural oxide film is formed when a protective film isformed on a metal film. Note that, it is regarded that the oxide layeris preferably about 3 nm thick when a W film is used. Further, it ispreferable to form the protective film without containing hydrogenthereby forming the oxide film faultlessly.

In accordance with the above-mentioned result, it is considerednecessary to form an oxide layer including the aforementioned metal (ametal oxide layer) on a metal layer for performing delamination.Particularly, when W is used for the metal film, it is found necessaryto perform heat treatment at least at 400° C. thereby crystallize anoxide layer with a thickness of about 3 nm. Further according to theexperimental result, it is found preferable to perform heat treatment atleast at 430° C. since the crystallization of the oxide layer spreadsall over.

Still further, it is found desirable that the aforementioned metal oxidelayer on the metal layer is formed when the protective film is formed,and the protective film is formed in a state where hydrogen is notincluded or where the hydrogen concentration is low. When a W film isgiven as a concrete example, it is found preferable to form theprotective film by sputtering using a source gas containing oxygen gas.

[Embodiment 3]

In this embodiment, a result of the observation of an oxide layer on theside of a substrate and the side of an amorphous semiconductor filmafter the separation using TEM will be shown.

A W film with a thickness of 50 nm is formed over a glass substrate bysputtering; a silicon oxide film with a thickness of 200 nm is formedthereafter by sputtering; subsequently, an silicon oxynitride film witha thickness of 100 nm is formed as a base film by plasma CVD; and anamorphous film with a thickness of 50 nm is formed as a semiconductorfilm likewise by plasma CVD. Then, heat treatment is performed at 500°C. for 1 hour and at 550° C. for 4 hours; a quartz substrate is adheredby using epoxy resin as a bond; and separation is performed by aphysical means. The TEM picture of the W film and the oxide layer on thesubstrate side at this point is shown in FIGS. 19A and 19B. The TEMpicture of the oxide layer and the silicon oxynitride film on thesemiconductor film is shown in FIGS. 20A and 20B.

In FIGS. 19A and 19B, the oxide layer remains nonuniformly on the metalfilm. Correspondingly, the oxide layer remains nonuniformly on thesilicon oxide film, as shown in FIG. 20. The two pictures demonstratethat the separation occurs in a layer of the oxide layer or at theinterfaces of the oxide layer, and that the oxide layer remainsnonuniformly cohered to the metal film and the silicon oxide film.

[Embodiment 4]

The result of the examining the composition of the oxide layer by usingXPS (X-ray photoelectron spectroscopy) is shown in this embodiment.

FIGS. 16A to 16C respectively show the results of samples A to C. InFIGS. 16A to 16C, the horizontal scale shows a depth direction (theinterior of the oxide layer is exposed by ion sputtering. The case where1 atomic % of tungsten is detected shall be pos. 1; the case where 2atomic % of tungsten is detected shall be pos. 2; and the case where 3atomic % of tungsten is detected shall be pos. 3)), the vertical scaleshows an occupied bond ratio (%).

When FIGS. 16A to 16C are compared, the relative ratio of tungsten (W)that is shown with a circle is higher in sample C compared with samplesA and B. Namely, sample C has a high proportion of tungsten and a lowproportion of tungsten oxide.

FIGS. 17A to 17F show the results of the standardization of the data ofFIGS. 16A to 16C. FIGS. 17A and 17D correspond to the result of sampleA. FIGS. 1733 and 17E correspond to the result of sample B. FIGS. 17Cand 17F correspond to the result of sample C. FIGS. 17A to 17C show agraph in which WO₃ shall be 1 and the occupied bond ratio of the othercompositions are standardized. FIGS. 17D to 17F show graphs in which WO₂shall be 1 and the occupied bond ratio of the other compositions arestandardized.

When FIGS. 17A to 17C are compared, the relative ratio of WO₂ that isshown with a cross is higher in sample C compared with samples A and B.Namely, sample C has a high proportion of O₂, and the proportion of WO₂becomes higher as the depth increases from pos. 1 to pos. 3. Further,sample C has a low proportion of WO_(x), and the proportion of WO₂ isfound to become lower as the depth increases from pos. 1 to pos. 3. WhenFIGS. 17D to 17F are compared, samples A and B have WO₂ contents of atleast 2% meanwhile sample C has a content of at most 2%. As apparentfrom the graph standardized on WO₃, sample C has higher proportion ofWO₂ compared with samples A and B.

FIGS. 18A to 18C show waveform analysis of bond energy and spectrumobserved when 1 atomic % of tungsten is detected (pos. 1), and theinterior of the oxide layer is exposed by ion sputtering. FIG. 18A showsthe result of sample A after sputtering process for four minutes and aquarter. FIG. 18B shows the result of sample B after sputtering processfor four minutes. FIG. 18C shows the result of sample C after sputteringprocess for five minutes. In FIGS. 18A to 18C, as to each of 4 states:W1 (tungsten W), W2 (tungsten oxide WO_(x), X is nearly 2), W3 (tungstenoxide WO_(x), 2<X<3), and W4 (tungsten oxide WO₃ or the like), the arearatio (%) is equivalent to the composition ratio.

Chart 1 shows the area ratios of the respective states W1 to W4 ofsamples A to C obtained from FIGS. 18A to 18C. The Chart 1 further showsa graph in which W2 and W3 are standardized on W4 by ratio. In the Chart1, samples A and B have 10% of the proportions of W1 while theproportion of sample C is high as 35%. Namely, sample C has a highproportion of tungsten and a low proportion of tungsten oxide. Accordingto the standardized value, it is found that sample C has a highproportion of W2 (WO₂) in the tungsten oxide, compared with samples Aand B.

Sample C has a high composition ratio of W2 (WO₂), and it is consideredthat the composition of the oxide layer is changed due to heattreatment. Accordingly, the composition of W4 (WO₃) is changed to W2(WO₂) or W3 (WO_(x)) and it is conceivable that separation occursbetween different crystal structures due to such differences of thecrystal structures.

CHART 1 W—Ox Standardized on WO3 in each State Sample Depth W1 W2 W3 W4W2 W3 W4 A Pos. 1 9.57 18.91 24.58 46.94 40.29% 52.36% 100.00% Pos. 212.54 18.83 22.19 46.44 40.55% 47.78% 100.00% Pos. 3 14.45 20.49 21.4943.57 47.03% 49.32% 100.00% B Pos. 1 11.32 19.68 22.42 46.58 42.25%48.13% 100.00% Pos. 2 14.57 19.15 21.91 44.38 43.15% 49.37% 100.00% Pos.3 15.46 21.2 22.17 41.18 51.48% 53.84% 100.00% C Pos. 1 35.51 16.3716.13 32 51.16% 50.41% 100.00% Pos. 2 37.44 17.2 15.8 29.57 58.17%53.43% 100.00% Pos. 3 40.94 17.43 13.3 28.33 61.52% 46.95% 100.00%

Next, the side of the substrate after the separation and the side of thesemiconductor film after the separation are measured with XPS. Themeasurements of the spectrum and the waveform analysis of the spectrumare shown in FIGS. 24A and 25B. Further, the XPS measurement of sample 1and the waveform analysis thereof are shown together to compare theoxide layer with the natural oxide film.

FIGS. 24A and 24B each show the spectrum of the separated surface whichis measured with XPS. FIG. 24A shows the spectrum of the separatedsurface of the semiconductor film side. FIG. 24B shows the spectrum ofthe separated surface of the substrate side.

Chart 2 shows the detected elements and a quantitative result obtainedfrom FIGS. 24A and 24B. The Chart 2 reveals that about ten times moretungsten remains on the side of the substrate than on the side of thesemiconductor film.

CHART 2 Oxygen Carbon Silicon Tungsten (O) (C) (Si) (W) Sample 1 41 20<1 38 Semiconductor 59 12 26 3 Film Side Substrate Side 51 20 less than29 detection limit

Subsequently, the waveform analysis of the spectrum on the side of thesemiconductor film is shown in FIG. 25A. The waveform analysis of thespectrum on the side of the substrate is shown in FIG. 25B. In FIGS. 25Aand 25B, as to each of 4 states: W1 (tungsten W), W2 (tungsten oxideWO_(x), X is nearly 2), W3 (tungsten oxide WO_(x), 2<X<3), and W4(tungsten oxide WO₃ or the like), the area ratio (%) is equivalent tothe composition ratio.

The spectrum of sample 1 in which a natural oxide film is formed isshown in the XPS measurement in FIG. 31. The waveform analysis of thespectrum is shown in FIG. 32. The area ratio of each state in sample 1and the intensity ratio of W2 and W3, which are standardized on W4 ineach sample are shown in Chart 3. Further, the measurement of thesurface of the semiconductor film side and the surface of the substrateside are shown together in the Chart 3.

CHART 3 Intensity Standardized on W4 in each State Sample W1 W2 W3 W4 W2W3 W4 Sample 1 69.54 6.42 1.03 23.01 27.90% 4.48% 100.00% Semi- 0 016.48 83.52 0.00% 19.73% 100.00% conductor Film Side after Delamina-tion Substrate 43.52 5.04 9.53 41.91 12.03% 22.74% 100.00% Side afterDelamina- tion

Further, FIG. 30A is a graph showing the intensity ratio of thecomponents in W1 to W4 based upon the charts 1 and 3. FIG. 30B is agraph showing the intensity ratio of W2 and W3, which are standardizedon W4.

The occupied bond ratio of the side of the semiconductor film after theseparation are as follows: W1 and W2 are 0%, W3 is 16%, and W4 is 84%;meanwhile on the substrate side, W1 is 44%, W2 is 5%, W3 is 10%, and W4is 46%. The occupied bond ratio of the natural oxide film in sample 1 isfound to be as follows: W1 is 70; W2 is 6; W3 is 1; and W4 is 23.

In addition, the proportion of W1 (tungsten) is found to be higher insample 1 compared with other samples. It is also found that theproportions of W2 to W4 (oxides) are low, and the proportion of W3 isconsiderably low.

The total amount of WO₂ on the sides of the semiconductor film and thesubstrate after the separation is found to be lower compared with WO₂ insample C. Hence, it can be considered that the state of the oxide layerbefore separation is energetically active (unstable), and W4 (WO₃) aswell as the natural oxide film become the main constituent therebystabilizing the state after the separation.

When sample C which can be separated and sample 1 in which the naturaloxide film is formed are compared using FIGS. 30A and 30B, sample C isfound to contain more W2 to W4 (oxides).

Accordingly, when the separation is performed at the interface betweenthe oxide layer and the metal film, at the interface between an oxidelayer and the silicon oxide film, or in a layer of the oxide layer, itis found that all of W1 (metal W) and W2 (WO_(x), X is nearly 2) remainon the substrate side; ⅔ of W4 (WO₃ and the like) remains on thesemiconductor film side; and ⅓ thereof remains on the side of thesubstrate. Further, the composition ratios of the oxide layer and thenatural oxide film are found to be different from each other. Thus, itis considered that the separation can easily be performed in a layer ofthe oxide layer, especially, at the interfaces between WO₂ and WO_(x) orbetween WO₂ and WO₃. Therefore, WO₂ does not remain on the side of thesemiconductor film and WO₂ adheres to the side of the substrate in theexperiment; however, it can be possible that WO₂ adheres to the side ofthe semiconductor film and no WO₂ exists on the side of the substrate.

[Embodiment 5]

In this embodiment, the result of secondary ion composition analysismethod (SIMS) performed against samples A to C will be described usingFIGS. 21 to 23.

When the profile of hydrogen in an amorphous silicon film is noted, thehydrogen concentration is approximately 1.0×10²² (atoms/cm³) in sample Aand B, whereas the hydrogen concentration is approximately 1.0×10²⁰(atoms/cm³) in sample C, almost twice as large as sample A and B. Whenthe profiles of hydrogen in silicon oxynitride film (SiON) and a siliconoxide film (SiO₂) are observed, it shows the nonuniform concentrationdistribution, such as a tendency to decrease in the vicinity of a depthat 0.2 μm in sample A and B. On the other hand, sample C shows theuniform concentration distribution in the direction of depth without atendency to decrease. Thus, more hydrogen exists in sample C than insamples A and B. According to the above result, it is considered thatthe ionization efficiency of hydrogen is different, and sample C has acomposition ratio of surface different from samples A and B.

Next, when the nitrogen concentration at the interface between thesilicon oxide film (SiO₂) and W film is noted, the nitrogenconcentration is approximately 1.0×10²¹ (atoms/cm³) in sample A and B,whereas the nitrogen concentration is approximately 6.5×10²¹ (atoms/cm³)in sample C, which is about 1 order of magnitude more than theconcentration in sample A and B. Accordingly, Sample C has a differentcomposition of the oxide layer at the interface between the siliconoxide film (SiO₂) and the W film compared with samples A and B.

[Embodiment 6]

In this embodiment, a light emitting device which is provided with a TFTmanufactured over a film substrate according to a delamination method ofthe present invention with reference to FIGS. 26A and 26B.

FIG. 26A shows a top view of a light emitting device; a signal linedriver circuit 1201, a scanning line driver circuit 1203, and a pixelarea 1202 are provided over a film substrate 1210.

FIG. 26B shows a cross section of a light emitting device taken alongthe line A-A′, and an oxide layer 1250 is provided over the filmsubstrate 1210 with a binding material 1240 therebetween. Note that, theoxide layer may be scattered instead of being formed as a layer on theback of the film substrate. When a W film is used as a metal film asdescribed in the above embodiment, the oxide layer serves as an oxidecomprising tungsten as a major component, WO₃, specifically.

A signal line driver circuit 1201 provided with a CMOS circuitcomprising an n-channel TFT 1223 and a p-channel TFT 1224, which isformed over the film substrate is shown. A TFT forming a signal linedriver circuit or the scanning line driver circuit may be formed from aCMOS circuit, a PMOS circuit, or an NMOS circuit. Further in thisembodiment, a built-in driver type wherein a signal line driver circuitand a scanning line drive circuit are formed over a substrate is shown;however, the circuits may be formed outside the substrate instead.

Further, an insulating film 1214 comprising 1212 a switching TFT 1221and a current controlling TFT, and further comprising an opening in apredetermined position, which covers the TFTs; a first electrode 1213connected to one of wirings of the current controlling TFT 1212; anorganic compound layer 1215 which is provided over a first electrode; alight emitting element 1218 comprising a second electrode 1216 which isprovided opposite to the first electrode; and a pixel area 1220comprising a protective layer 1217 which is provided to preventdeterioration of a light emitting element caused by water or oxygen, areshown.

Owing to the structure wherein the first electrode 1213 contacts a drainof the current controlling TFT 1212, it is desirable that at least thebottom of the first electrode 1213 shall be formed from a material thatcan form an ohmic contact with a drain region of the semiconductor film,or a material having a high work function in the surface comprising anorganic compound. For example, when a three-layer structure of atitanium nitride film/a film comprising aluminum in major proportions/atitanium nitride film, is employed, the resistance as a wiring is lowand the performance of making a good ohmic contact can be obtained.Further, the first electrode 1213 may be a single layer of a titaniumnitride film, or a lamination having more than three layers.Furthermore, a light emitting device of a double side emission type canbe manufactured by employing a transparent conductive film as the firstelectrode 1213.

The insulating film 1214 may be formed from an organic resin film or aninsulating film comprising silicon. A positive photosensitive acrylicfilm is used here for the insulating film 1214.

It is preferable that the top edge and bottom edge of the insulatingfilm 1214 is formed so as to have a curved surface with a curvature,thereby improving the coverage of a light emitting layer comprising anorganic compound and the second electrode. For example, when a positivephotosensitive acrylic film is employed for the insulating film 1214, itis preferable that the top edge of the insulating film 1214 solely has acurved surface with a curvature (0.2 μm to 3 μm). Further, whichever ofa negative type that becomes insoluble in an etchant with light or apositive type that becomes soluble in an etchant with light can be used.

Further, the insulating film 1214 may be covered with a protective film.The protective film may be an aluminum nitride film obtained by a filmformation system using sputtering (DC system or RF system) or remoteplasma; an aluminum oxynitride film; an insulating film such as asilicon nitride film comprising silicon nitride or silicon oxynitride inmajor proportions; or a thin film comprising carbon in majorproportions. It is desirable that the film thickness of the protectivemay be thin as possible so that light can transmit through theprotective film.

A layer including an organic compound in which the luminescence of R, G,and B are obtained by applying an evaporation method with the use of aevaporation mask or ink-jetting is selectively formed over the firstelectrode 1213. Further the second electrode is formed over the layerincluding an organic compound 1215.

When the light emitting element 1218 shall emit white light, a colorfilter formed of a colored layer and a black mask needs to be formed.

The second electrode 1216 is connected to a connection wiring 1208through an opening (a contact) provided over the insulating film 1214 ina connection area. The connection wiring 1208 is connected to a flexibleprinted circuit (FPC) 1209 by an anisotropic conductive resin (ACF). Avideo signal and a clock signal are received from an FPC 1209 which isto be an external input port. Only the FPC is illustrated here; however,a printed wiring board (PWB) may be attached to the FPC.

When the FPC is connected by applying pressure or heat with the use ofan ACF, it is noted that a crack due to the flexibility of a substrateor softening caused by heat should be prevented from generating. Forexample, a substrate with high hardness may be disposed as an assistanceon a part of the film substrate 1210, opposite to the part where the FPCis adhered.

The marginal portion of a substrate is provided with a sealing material1205, and the substrate is pasted to a second film substrate 1204, andencapsulated. An epoxy resin is preferably used as the sealing material1205.

In this embodiment, a substrate formed of FRP (Fiber-ReinforcedPlastics), PVF (polyvinyl fluoride), mylar, polyester, acryl, or thelike besides a glass substrate and a quartz substrate may be employed asa material for forming the second film substrate 1204.

Although it is not illustrated, the film substrate may be covered with abarrier film comprising an organic material such as polyvinyl alcohol orethylene vinyl alcohol copolymer; an inorganic material such aspolysilazane, aluminum oxide, silicon oxide, or silicon nitride; or alamination of those, for preventing water or oxygen from penetratingthrough the film substrate.

A protective layer may be provided over the film substrate to protectfrom a medicine in a manufacturing process. An ultraviolet curable resinor a thermosetting resin can be used for the protective layer.

As described above, a light emitting device comprising a TFT providedover a film substrate is completed. The light emitting device comprisinga TFT according to the present invention is hard to be broken even if itdrops, and lightweight. A film substrate enables the enlargement of alight emitting device as well as mass production.

[Embodiment 7]

A liquid crystal display device comprising a formed over a filmsubstrate by a delamination method according to the present inventionwill be described with reference to FIGS. 27A and 27B in thisembodiment.

FIG. 27A shows a top view of a liquid crystal display device; a signalline driver circuit 1301, a scanning line driver circuit 1303, and apixel area 1302 are provided over a first film substrate 1310.

FIG. 27B shows a cross section of a liquid crystal display device takenalong the line A-A′, and an oxide layer 1350 is formed over a filmsubstrate 1310 with a binding material 1340 in between. Note that, theoxide layer may be scattered instead of being formed as a layer on theback of the film substrate. When a W film is used as a metal film asdescribed in the above embodiment, the oxide layer serves as an oxidecomprising tungsten as a major component, WO₃, specifically.

A signal line driver circuit 1301 provided with a CMOS circuitcomprising an n-channel TFT 1323 and a p-channel TFT 1324 is formed overthe film substrate. A TFT forming a signal line driver circuit or ascanning line driver circuit may be formed from a CMOS circuit, a PMOScircuit, or a NMOS circuit. Further in this embodiment, a built-indriver type wherein a signal line driver circuit and a scanning linedrive circuit are formed over a substrate is shown; however, thecircuits may be formed outside the substrate.

Further, a pixel area provided with an interlayer insulating film 1314comprising a switching TFT 1321 and a retention volume 1312, and furthercomprising an opening in a predetermined position, which covers the TFTsis shown.

An oriented film 1317 is provided over the interlayer insulating film1314, and is treated with rubbing.

A second film substrate 1304 is prepared as a counter substrate. Thesecond film substrate 1304 is provided with a color filter of RGB 1330,a counter electrode 1316, and an oriented film 1317 that is treated withrubbing, in an area partitioned into matrix form with resin or the like.

A polarizer 1331 is provided over the first and second film substrates,and is adhered with a sealing material 1305. And a liquid crystalmaterial 1318 is injected between the first and second film substrates.It is not illustrated; however, a spacer is provided appropriately tomaintain a gap between the first and the second film substrates.

Although it is not illustrated, the film substrate may be covered with abarrier film comprising an organic material such as polyvinyl alcohol orethylene vinyl alcohol copolymer; or an inorganic material such aspolysilazane, or silicon oxide; or a lamination of those, for preventingwater or oxygen from penetrating through the film substrate.

A protective layer may be provided to protect from a medicine in amanufacturing process. An ultraviolet curable resin or a thermosettingresin can be used for the protective layer.

Like in FIGS. 26A and 26B, a wiring and a flexible printed circuit (FPC)are connected together by an anisotropic conductive resin (ACF), andreceive a video signal and a clock signal. Note that, a connection withan FPC by applying pressure or heat needs attention to prevent a crackfrom generating.

As described above, a liquid crystal display device comprising a TFTprovided over a film substrate is completed. The liquid crystal displaydevice comprising a TFT according to the present invention is hard to bebroken even if it drops, and lightweight. A film substrate enables theenlargement of a liquid crystal display device as well as massproduction.

[Embodiment 8]

An embodiment according to the present invention will be described withreference to FIG. 28. A panel having a pixel area, a driver circuit forcontrolling the pixel area, a memory circuit, and a CPU comprising acontrol unit and an arithmetic unit over on insulating surface will beexplained in this embodiment.

FIG. 28 shows the appearance of a panel. The panel has a pixel area 3000wherein plural pixels are arranged in matrix over a substrate 3009. Ascanning line driver circuit 3001, a scanning line driver circuit 3001for controlling the pixel area 3000, and a signal line driver circuit3002 are provided at the periphery of the pixel area 3000. In the pixelarea 3000, an image is displayed according to a signal supplied from thedriver circuit.

The counter substrate may be provided only over the pixel area 3000 andthe driver circuits 3001 and 3002, or may be provided over the entiresurface alternatively. Note that, it is preferable that the CPU 3008that may generate heat be provided with a heat sink contiguously.

Further, the panel also has a VRAM 3003 (video random access memory) forcontrolling the driver circuits 3001 and 3002, and decoders 3004 and3005 at the periphery of the VRAM 3000. In addition, the panel has a RAM(random access memory) 3006, a decoder 3007 at the periphery of the RAM3006, and the CPU 3008.

All elements forming a circuit over the substrate 3009 are formed of apolycrystalline semiconductor (polysilicon) that has higher field-effectmobility and higher ON current than that of an amorphous semiconductor.Therefore, a plurality of circuits can be formed into an integratedcircuit over one insulating surface. A pixel area 3001, driver circuits3001 and 3002, and another circuit are formed over a support substratefirst, and separated by the delamination method according to the presentinvention, then, pasted with each other thereby achieving an integratedstructure over there flexible substrate 3009. The structure of theplural pixels in the pixel area is, but not exclusively, formed byproviding SRAM to each of the plural pixels. Thus, VRAM 3003 and RAM3006 may be omitted.

[Embodiment 9]

The present invention can be applied to various electronic devices.Given as examples as the electronic devices: a personal digitalassistance (a cellular phone, a mobile computer, a portable gamemachine, an electronic book, or the like), a video camera, a digitalcamera, a goggle type display, a display, a navigation system, and thelike. FIGS. 29A to 29E are views showing these electronic devices.

FIG. 29A shows a display including a frame 4001, a sound output unit4002, a display unit 4003, and the like. The present invention is usedto the display unit 4003. The display includes all information displayssuch as a personal computer, a TV broadcasting, and an advertisementdisplay.

FIG. 29B shows a mobile computer having a main body 4101, a stylus 4102,a display unit 4103, an operation button 4104, an external interface4105, and the like. The present invention is used to the display unit4103.

FIG. 29C shows a game machine including a main body 4201, a display unit4202, an operation button 4203, and the like. The present invention isused to the display unit 4202. FIG. 29D is a cellular phone including amain body 4301, a sound output unit 4302, a sound input unit 4303, adisplay unit 4304, an operation switch 4305, an antenna 4306, and thelike. The present invention is used to the display unit 4304.

FIG. 29E shows a electronic book reader including a display unit 4401and the like. The present invention is used to the display unit 4401.

Since the application range of the present invention is extremely large,the present invention can be applied to various electronic devices inall fields. Especially, the present invention that enables devices to bethinner and/or lighter is remarkably effective for the electronicdevices illustrated in FIGS. 29A to 29E.

By employing a delamination method according to the present invention, aTFT or the like can be formed over a flexible film substrate achievinghigh yield since separation can be performed in the whole surface.Further, a burden caused by a laser or the like are not placed on a TFTin the present invention. Thus, a light emitting device, a display unitof a liquid crystal display device, or the like, which has the nil andthe like can be made thin, hard to be broken even if it drops, andlightweight. Further, display on a curved surface or in odd-shape isenabled.

A TFT on a film substrate, which is formed according the presentinvention can achieve the enlargement of display units as well as massproduction. The present invention enables the recycling of a firstsubstrate on which a TFT or the like to be formed before transferring,and achieves reducing costs of a semiconductor film by employing alow-cost film substrate.

What is claimed is:
 1. A method for manufacturing a display devicecomprising the steps of: forming a metal film comprising a metal over afirst substrate; forming a first insulating film comprising oxygen overthe metal film, wherein an oxide layer comprising the metal is formed byoxidation at an interface between the metal film and the firstinsulating film; forming a semiconductor element over the firstinsulating film; forming a second substrate over the semiconductorelement; and separating the semiconductor element adhered to the secondsubstrate from the first substrate.
 2. The method according to claim 1,wherein the metal film comprises one material selected from Ti, Ta, Mo,Nd, Ni, Co, Zr, Zn, Ru, Rh, Pd, Os, and Ir.
 3. The method according toclaim 1, further performing a heat treatment at 400° C. or more afterforming the oxide layer.
 4. The method according to claim 1, wherein theseparation occurs in the oxide layer.
 5. The method according to claim1, wherein the separation occurs between the oxide layer and the metalfilm.
 6. The method according to claim 1, further comprising steps of:transferring the semiconductor element onto a third substrate;separating the second substrate from the semiconductor element; andforming a fourth substrate over the semiconductor element after theseparation of the second substrate, wherein the third substrate and thefourth substrate are flexible substrates.
 7. The method according toclaim 1, wherein a film thickness of the oxide layer is 0.1 nm to 5 nm.8. The method according to claim 1, wherein the oxide layer includescrystals arranged in one direction.
 9. The method according to claim 1,wherein the semiconductor element comprises a semiconductor film. 10.The method according to claim 1, further forming a second insulatingfilm comprising nitrogen over the first insulating film.
 11. A methodfor manufacturing a display device comprising the steps of: forming ametal film comprising tungsten over a first substrate; forming a firstinsulating film comprising oxygen over the metal film, wherein an oxidelayer comprising the tungsten is formed by oxidation at an interfacebetween the metal film and the first insulating film; forming asemiconductor element over the first insulating film; forming a secondsubstrate over the semiconductor element; and separating thesemiconductor element adhered to the second substrate from the firstsubstrate.
 12. The method according to claim 11, wherein the separationoccurs in the oxide layer.
 13. The method according to claim 11, whereinthe separation occurs between the oxide layer and the metal film.
 14. Amethod for manufacturing a display device comprising the steps of:forming a metal film comprising tungsten over a first substrate;performing a plasma treatment to form an oxide layer over the metalfilm; forming a first insulating film comprising oxygen over the metalfilm; forming a semiconductor element over the first insulating film;forming a second substrate over the semiconductor element; andseparating the semiconductor element adhered to the second substratefrom the first substrate.
 15. The method according to claim 14, whereinthe separation occurs in the oxide layer.
 16. The method according toclaim 14, wherein the separation occurs between the oxide layer and themetal film.
 17. A method for manufacturing a display device comprisingthe steps of: forming a metal film comprising tungsten over a firstsubstrate; forming a first insulating film comprising oxygen over themetal film, wherein an oxide layer comprising the tungsten is formed byoxidation at an interface between the metal film and the firstinsulating film; performing a heat treatment to increase a proportion ofWO₂ in the oxide layer; forming a semiconductor element over the firstinsulating film; forming a second substrate over the semiconductorelement; and separating the semiconductor element adhered to the secondsubstrate from the first substrate.
 18. The method according to claim17, wherein the separation occurs in the oxide layer.
 19. The methodaccording to claim 17, wherein the separation occurs between the oxidelayer and the metal film.